Mechanical and chemical-mechanical planarizing processes (collectively "CMP") are used in the manufacturing of electronic devices for forming a flat surface on semiconductor wafers, field emission displays and many other microelectronic-device substrate assemblies. CMP processes generally remove material from a substrate assembly to create a highly planar surface at a precise elevation in the layers of material on the substrate assembly.
FIG. 1 schematically illustrates an existing web-format planarizing machine 10 for planarizing a substrate 12. The planarizing machine 10 has a support table 14 with a top-panel 16 at a workstation where an operative portion (A) of a planarizing pad 40 is positioned. The top-panel 16 is generally a rigid plate to provide a flat, solid surface to which a particular section of the planarizing pad 40 may be secured during planarization.
The planarizing machine 10 also has a plurality of rollers to guide, position and hold the planarizing pad 40 over the top-panel 16. The rollers include a supply roller 20, first and second idler rollers 21a and 21b, first and second guide rollers 22a and 22b, and a take-up roller 23. The supply roller 20 carries an unused or pre-operative portion of the planarizing pad 40, and the take-up roller 23 carries a used or post-operative portion of the planarizing pad 40. Additionally, the first idler roller 21a and the first guide roller 22a stretch the planarizing pad 40 over the top-panel 16 to hold the planarizing pad 40 stationary during operation. A motor (not shown) drives at least one of the supply roller 20 and the take-up roller 23 to sequentially advance the planarizing pad 40 across the top-panel 16. As such, clean pre-operative sections of the planarizing pad 40 may be quickly substituted for used sections to provide a consistent surface for planarizing and/or cleaning the substrate 12.
The web-format planarizing machine 10 also has a carrier assembly 30 that controls and protects the substrate 12 during planarization. The carrier assembly 30 generally has a substrate holder 32 to pick up, hold and release the substrate 12 at appropriate stages of the planarizing cycle. A plurality of nozzles 33 attached to the substrate holder 32 dispense a planarizing solution 44 onto a planarizing surface 42 of the planarizing pad 40. The carrier assembly 30 also generally has a support gantry 34 carrying a drive assembly 35 that translates along the gantry 34. The drive assembly 35 generally has an actuator 36, a drive shaft 37 coupled to the actuator 36, and an arm 38 projecting from the drive shaft 37. The arm 38 carries the substrate holder 32 via another shaft 39 such that the drive assembly 35 orbits the substrate holder 32 about an axis B--B offset from a center point C--C the substrate 12.
The planarizing pad 40 and the planarizing solution 44 define a planarizing medium that mechanically and/or chemically-mechanically removes material from the surface of the substrate 12. The web-format planarizing machine 10 typically uses a fixed-abrasive planarizing pad in which abrasive particles are fixedly bonded to a suspension material. In fixed-abrasive applications, the planarizing solution is generally a "clean solution" without abrasive particles because the abrasive particles are fixedly distributed across the planarizing surface 42 of the planarizing pad 40. In other applications, the planarizing pad 40 may be a non-abrasive pad composed of a polymeric material (e.g., polyurethane), a resin, or other suitable materials without abrasive particles. The planarizing solutions 44 used with the non-abrasive planarizing pads are typically CMP slurries with abrasive particles and chemicals to remove material from a substrate.
To planarize the substrate 12 with the planarizing machine 10, the carrier assembly 30 presses the substrate 12 against the planarizing surface 42 of the planarizing pad 40 in the presence of the planarizing solution 44. The drive assembly 35 then orbits the substrate holder 32 about the offset axis B--B to translate the substrate 12 across the planarizing surface 42. As a result, the abrasive particles and/or the chemicals in the planarizing medium remove material from the surface of the substrate 12.
CMP processes should consistently and accurately produce a uniformly planar surface on the substrate assembly to enable precise fabrication of circuits and photo-patterns. During the fabrication of transistors, contacts, interconnects and other components, many substrate assemblies develop large "step heights" that create a highly topographic surface across the substrate assembly. To enable the fabrication of integrated circuits with high densities of components, it is necessary to produce a highly planar substrate surface at several stages of processing the substrate assembly because non-planar substrate surfaces significantly increase the difficulty of forming sub-micron features. For example, it is difficult to accurately focus photo-patterns to within tolerances approaching 0.1 .mu.m on non-planar substrate surfaces because sub-micron photolithographic equipment generally has a very limited depth of field. Thus, CMP processes are often used to transform a topographical substrate surface into a highly uniform, planar substrate surface.
In the competitive semiconductor industry, it is also highly desirable to have a high yield in CMP processes by quickly producing a uniformly planar surface at a desired endpoint on a substrate assembly. For example, when a conductive layer on a substrate assembly is under-planarized in the formation of contacts or interconnects, many of these components may not be electrically isolated from one another because undesirable portions of the conductive layer may remain on the substrate assembly over a dielectric layer. Additionally, when a substrate assembly is over planarized, components below the desired endpoint may be damaged or completely destroyed. Thus, to provide a high yield of operable microelectronic devices, CMP processing should quickly remove material until the desired endpoint is reached.
To accurately create highly planar substrate surfaces at the desired endpoint, the particle size distribution of planarizing slurries with abrasive particles should: (1) be consistent from one batch of slurry to another; and (2) have small particle sizes. For example, many slurries have abrasive particles with individual particle sizes of approximately 10-250 nm. One problem with CMP processing, however, is that individual abrasive particles may agglomerate into larger abrasive elements that have a plurality of abrasive particles. The formation of such abrasive elements affects the consistency of the slurry because the extent that the particles agglomerate varies from one batch of slurry to another, or even within a single batch of slurry. Additionally, large abrasive elements may scratch the wafer and produce defects, or even smaller abrasive elements may effectively increase the particle size distribution of the slurry. Thus, the agglomeration of abrasive particles into larger abrasive elements is a serious problem for fabricating very small electronic components with CMP processes.
One existing technique to reduce agglomerations of abrasive particles is to select abrasive particles that repel each other. For example, aluminum oxide particles have a sufficient electrical charge at the pH levels of most slurries to substantially repel one another and prevent the particles from agglomerating. Ceria oxide (ceria) particles, on the other hand, do not inherently repel one another because they are generally isoelectric at the pH levels of most ceria based slurries. Accordingly, another technique to reduce agglomerations of abrasive particles is to add a dispersant to the slurry that prevents the ceria particles from agglomerating with one another. A typical dispersant covers the ceria particles with a compound that prevents individual particles or agglomerations of particles from becoming attached together. Although using appropriately charged particles and/or dispersants reduces agglomerations of abrasive particles in slurries, many slurries still suffer from the formation of a large number of agglomerations that increase the particle size distribution of the slurries.
In light of the problems of maintaining a consistent distribution of small abrasive particles and abrasive elements in the slurries, slurries are generally transported to the device manufacturers in two separate components. The first slurry component typically includes water and the abrasive particles. The second slurry component typically includes a liquid and other chemicals, such as surfactants, dispersants, oxidants, etc. The device manufacturers generally mix the first and second slurry components together to form a batch of slurry within forty-eight hours of using the slurry in a planarizing machine.
Although mixing the abrasive particles with the other slurry components shortly before using the slurry reduces agglomerations of abrasive particles, many intermediate and large agglomerations of abrasive particles still form in the mixed batches of slurry prior to being used in the planarization machines. Thus, merely mixing the first and second slurry components together to form a batch of slurry shortly before using the slurry does not alleviate the problems associated with agglomerations of abrasive particles.
One method to reduce the agglomerations of abrasive particles in a slurry is to impart sonic energy to the first and second slurry components after they have been mixed together. By sonicating both the first and second slurry components of the mixed slurry just before the mixed slurry is deposited onto the polishing pad, the sonic energy breaks apart the abrasive elements to reduce the particle size distribution of the slurry. Although sonicating the mixed slurry as it flows to the planarizing tool reduces the particle size distribution in several applications, the sonic energy often does not separate agglomerated abrasive particles in slurries containing a dispersant because the dispersant encapsulates and protects the abrasive elements as the first and second slurry components are mixed together prior to sonicating the mixed slurry. Thus, even sonicating a slurry mixture just before it flows to the planarizing machine may not effectively break apart the abrasive elements.